The proposed research consists of several projects that are designed to investigate the structure and function of Fc receptors on different types of lymphocytes: 1. The nature of the lymphocytes that have Fc receptors for IgE in both man and rats will be characterized. The number of these cells in normals and patients with allergies as well as other diseases will be determined. 2. The chemical nature of isolated Fc receptors for IgE from cultured human lymphoblastoid cells Wil-2WT will be investigated. 3. The affinity constants of immunoglobulins of various classes and subclasses to subpopulations of lymphocytes (T,B and K cells) will be determined. 4. The type of human K lymphocytes that become activated by Fc fragments of IgG2 and IgG4 will be investigated as well as the mechanism of activation. 5. It will be studied if monomeric or aggregated IgG and IgE inhibit polyvalent B cell activation by pokeweed mitogen.